U.S. Pat. No. 4,397,270 (=JP-A 55-137305) discloses a VVA apparatus. FIG. 26 illustrates this known VVA apparatus. It includes a drive shaft 2, a control shaft 3 with axially spaced eccentric control cams 4, and a pivot structure 9. The pivot structure 9 supports valve operating (VO) cams 8 for pivotal motion above valve lifters 7 of cylinder valves 6 of an internal combustion engine 1. Each VO cam 8 has a cam surface in sliding contact with the associated valve lifter 7 at a top or upper surface 7a thereof. Springs 10 are mounted for the VO cams 8, respectively. Each of the springs 10 biases one of the corresponding rocker cams 5 toward its rest position where the associated cylinder valve closes. Rocker arms 5 operate the VO cams, respectively. The eccentric control cams 4, which are in rotary unison with the control shaft 3, bear the rocker arms 5, respectively. An axis of each of the eccentric control cams 4 serves as the center of pivot of the corresponding one of the rocker arms 5. Drive cams 2a fixed to the drive shaft 2 operate the rocker arms 5, respectively. An electronic controller is provided. At one end portion 5a, each of the rocker arms 5 is in abutting contact with the associated drive cam 2a. At the other end portion 5b, the rocker arm 5 is abutting contact with a shoulder 8b of the associated VO cam 8. Sensors on the engine send information on engine speed, engine load, vehicle speed, and coolant temperature to the controller. At a predetermined switchover point, the controller sends a signal to an actuator for the control shaft 3. As the actuator turns the control shaft 3, the eccentricity of each of the eccentric cams 4 with respect to an axis of the control shaft 3 changes. This alters the position of pivot center of the rocker arms 5 relative to the position of pivot center of the VO cams 8. This causes variation in valve timing and lift of each of the cylinder valves 6.
FIG. 27 illustrates a conventional rotary VO cam 51 that rotates through 360 degrees to move the associated cylinder valve via a valve lifter 61. The valve lifter 61 has an upper surface 61a in sliding contact with a cam face of the VO cam 51. FIG. 28 illustrate characteristics presented by the rotary VO cam 51 of FIG. 27. FIG. 28 shows a buffer speed section (or a ramp speed section) .theta.r, a positive acceleration section .theta..sub.1 and a negative acceleration section .theta..sub.2. During this negative acceleration section .theta..sub.2, valve lift deceleration takes place to provide smooth variation in the neighborhood of the maximum lift. The two-dot chain line denotes a speed y' of lift with respect to cam angle. The speed y' become the maximum y'.sub.max at the boundary between the positive acceleration section .theta..sub.1 and the negative acceleration section .theta..sub.2.
Range of distance by which the rotary VO cam 51 slides on the flat upper surface 61 of the valve lifter 61 is called a travel distance t. This travel distance t can be expressed as EQU t=dy/d .theta..
Thus, the travel distance t is equal to the speed y.
Let us now evaluate on the pivotal VO cam 8 of the prior art VVA apparatus illustrated in FIG. 26. In the case of the pivotal cam 8, a sufficiently large valve lift must be produced within a relatively small angle through which the VO cam 8 can pivot. This inevitably requires an increased y'.sub.max. This increased y'.sub.max requires increased travel distance t. Thus, there is the potential risk that the cam nose of the VO cam 8 might come into abutting engagement with the outer edge 7a after having disengaged from the flat upper surface 7a of the valve lifter 7.
Thus, it is demanded to arrange the bearing structure 9 of the VO cam 8 at a location sufficiently deviated from the centerline of cylinder valve and/or increase the diameter of the upper surface 7a of the valve lifter 7. The known VVA apparatus as illustrated in FIG. 28 therefore has restrictions in layout of its component parts. Furthermore, the known VVA apparatus is bulky and difficult to reduce total weight. If the speed y' is reduced to decrease the travel distance t, the maximum lift y.sub.max becomes below a satisfactory level.
An object of the present invention is to provide a VVA apparatus wherein both the travel distance t and the maximum lift y.sub.max are tamed to provide improved performance without any modification in interconnection among major components parts.